Heart defects constitute a major proportion of all congenital anomalies in newborns. Five to ten percent of these are septal and valve lesions which result from abnormal development of the precursor endocardial cushion tissue. This proposal focuses on the role of the extracellular matrix (ECM) in influencing the development of the embryonic heart valve and septal tissue, and how abnormalities in the structure or metabolic processing of specific ECM macromolecules may lead to specific pathologies. In addition to their structural roles in maintaining tissue integrity, ECM macromolecules appear to have dramatic effects on the behavior of specific cells during morphogenesis and tissue remodeling. This is particularly so in the developing endocardial cushions, where hyaluronate has received considerable attention in this regard, as have more recently, heparin sulfate proteoglycan, and glycoproteins. However, little is known as to how these macromolecules influence cell behavior or how the cells regulate the levels of these components in the ECM. To resolve these questions, we propose the following research plan: 1. A combined biochemical, immunochemical, and immunohistochemical approach to determine the role of hyaluronate in endocardial cushion development. Experiments aim to reveal the effects of hyaluronate on cushion cell proliferation and migration, to establish the relationship of hyaluronate size to its biological influences, and to determine how cells regulate the levels of extracellular hyaluronate. The latter studies will include the use of anti-hyaluronidase monoclonal antibodies. 2. An immunohistochemical approach to define the macromolecular components present in the endocardial matrix, their cellular source of synthesis, and the changes which occur in these components as development proceeds. 3. An immunohistochemical approach to determine the embryonic origin of cushion and mural endocardial cells in an attempt to reveal if differences in their developmental origin might account for their different responses to the extracellular matrix.